The present invention relates generally to a mobile communication system; and, more particularly, to a method, a switching node and a program for the handover of user equipment from a first base station subsystem controlled by a first switching node to a second base station subsystem controlled by a second switching node, and which provides the second base station subsystem with the required parameters for the handover.
When a user moves through a cellular communication system or requests a connection with different parameters it is often necessary to perform a handover of user equipment from a first base station subsystem in the communication system to a second base station subsystem. In the handover, a connection of the user equipment, e.g. a mobile phone, to the second base station subsystem is established and the connection to the first base station subsystem is generally released. Customary base station subsystems comprise one or several base stations for the wireless connection to the user equipment and a base station controller which controls the base stations and connects them to the core network of the communication system. The controller can be for example a base station controller (BSC) as defined in GSM or a radio network controller (RNC) for a base station subsystem according to the UMTS standard.
A base station subsystem, i.e. the controller, is in turn controlled by a switching node, e. g. a mobile switching center (MSC). A switching node is connectable or permanently connected to a base station subsystem with a protocol corresponding to the connection specification used between the base station subsystem and the user equipment. For a connection according to the UMTS standard, the RANAP (Radio Access Network Application Protocol) protocol is exchanged over the lu interface between the switching node and the controller in the base station subsystem while GSM subsystems use the BSSMAP (Base Station System Management Application Part) protocol over the A interface to the switching node. Correspondingly, an MSC in a GSM system generally exchanges only BSSMAP messages with a BSC while a 3G MSC in a UMTS system processes RANAP messages. A 3G MSC can normally also process BSSMAP messages and can be connected to a BSC according to GSM specifications over an A interface.
Frequently, a handover is necessary between base station subsystems which are controlled by different switching nodes. In this case, the switching nodes exchange messages to perform the handover procedure of the user equipment from the first base station subsystem to the second base station subsystem which is controlled by a second switching node. The messages are described in 3GPP technical specification 3G TS 29.010 V 3.0.0. They encapsulate a set of parameters used in the protocol between the switching nodes and the base station subsystem. The set is necessary to perform the handover and comprises an identification of the connection for which the handover is to be executed and parameters which are required to provide defined services to the user. The necessary parameters for different types of connections, especially speech and data connections can differ.
For communication between switching nodes and transfer of the parameter set, MAP (Mobile Application Part) messages as described in 3GPP Technical Specification 29.002 V 3.2.0 are suitable which allow to encapsulate BSSMAP messages. The parameter set is generated in the first switching node. serving the user equipment from a message which is transmitted from the user equipment to the first switching node. The latter message which is generally in the DTAP (Direct Transfer Application Part) protocol is forwarded by the first base station subsystem without evaluation.
On the interface to the base station subsystem, user equipment is often capable of a connection according to at least two connection specifications, e.g. according to the standards for the GSM and UMTS air interfaces. User equipment which can set up connections according to further standards, e.g. DECT (Digital European Cordless Telecommunications) or WLAN (Wireless Local Area Network), in addition to at least one of these or other connection specifications is also possible. Correspondingly, many communication systems comprise base station subsystems for connection to the user equipment wherein different subsystems use different connection specifications on the interface to the user equipment.
The necessary parameters in the set used in the protocol between a switching node and a base station subsystem are dependent on the connection specification between the user equipment and the base station subsystem because the protocol is adapted to the connection specification, i.e. the interface between base station and user equipment. The messages over a specific interface comprise mandatory fields which have to be filled in from the parameter set. Parameter sets for different protocols comprise different mandatory parameters. Consequently, the connection will be lost in an intersystem handover if the parameters necessary for the handover are not provided. This problem is aggravated if the movements of the user require a repeated handover between connections of different specifications. An additional problem is that the handover between different switching nodes requires a considerable time which can also lead to a disruption of the connection.
It is therefore an object of the present invention to obviate these disadvantages and provide a method for the handover between base station subsystems controlled by different switching nodes which provides the target subsystem with the required parameters for the handover. It is a further object, to provide a method which is simple and easy to implement. It is still another object to provide a method which performs the handover in a short time.
According to the invention, a method is provided for a handover of user equipment in a mobile communication system from a first base station subsystem controlled by a first switching node to a second base station subsystem controlled by a second switching node. The user equipment is capable of connections to base stations according to at least two connection specifications, and the communication system comprises different base station subsystems for connection to the user equipment using one of the at least two connection specifications. The second switching node controls the second base station subsystem using a protocol corresponding to the connection specification used on an interface between the base station subsystem and the user equipment. The switching nodes exchange messages in the handover which encapsulates a parameter set for one of the protocols, the parameter set is generated in the first switching node from a message which is transmitted from the user equipment to the first switching node, the first switching node detects out of a message transmitted from the user equipment whether the user equipment is capable of connection according to different specifications, and the first switching node selects the parameter set encapsulated in the message to the second switching node according to the protocol for the connection of the second switching node to the second base station subsystem.
In the proposed method, the first switching node determines from a message transmitted from the user equipment whether the user equipment is capable of connections according to different specifications. The user equipment can be for example a mobile phone, a personal digital assistant (PDA) or a palmtop computer which is connectable to the communication network according to different specifications, e.g. with a GSM or UTRAN (UMTS Terrestrial Radio Access Network) and a WLAN connection. Preferably, the evaluated message is a DTAP message from the user equipment to the first switching node during the idle mode of the user equipment before a connection or during the establishment of the connection. The evaluated message can be the message which is used for the generation of the parameter set transmitted in the handover procedure.
The first switching node selects the parameter set encapsulated in the message to the second switching node according to the protocol for the connection of the second switching node to the target base station subsystem, i.e. the subsystem to which the handover procedure of the user equipment is performed. If both base station subsystems use the same connection specification on the interface to the user, the protocol corresponding to this connection specification is selected and encapsulated in the messages between the switching nodes. Else the selection depends on the implementations described below. It is either possible to generate only the selected parameter set or to generate different parameter sets and select one of them for encapsulation.
It is often suitable to encapsulate the set of parameters in a specific message type, e.g. a RANAP message, for which the encapsulation into a MAP message is not defined in present standards. However, because a MAP message can enclose different message types it is sufficient for this purpose to amend the list of allowed message types for encapsulation in MAP messages by the specific type. The message type contained in the MAP message is indicated by a data field in the header of the MAP message which can then be set to the specific message type enclosed from the amended list. In this way, also RANAP messages or further message specifications can be encapsulated into a MAP message and processed by the target switching node.
The proposed method has the advantage that it is simple to implement and ensures a safe execution of the handover procedure. The selection of the parameter set ensures that the target base station subsystem is provided with the parameters necessary for the establishment of the connection to the user equipment. Disruptions of a connection due to a handover of the user equipment between base station subsystems using different connection specifications are avoided.
In a preferred embodiment, the first switching node serving the user equipment generates parameter sets for different protocols. Advantageously, a parameter set corresponding to every connection specification to which a handover is possible is generated. The parameter sets are stored in the first switching node. Upon the request of a handover to a target base station subsystem controlled by the target node, the parameter set according to the target base station subsystem is encapsulated in the message and sent to the target switching node. In this way, the time for the execution of the handover is reduced and the risk for a disruption of a connection during handover is diminished.
Preferably, the first switching node serving the user equipment generates all parameter sets from a message transmitted from the user equipment. In this way, a mapping between parameter sets for different types of messages, for example BSSMAP and RANAP messages, is avoided. An advantage is that those parameters which are defined only in one of the messages are not lost during the mapping procedure and need not to be replaced by default values.
Alternatively, a switching node maps the parameter set for one protocol from a parameter set for a different protocol. The mapping can either be performed in the switching node first serving the user or in the target switching node of a handover. The latter embodiment is preferable if only a small number of nodes is connected to base station subsystems of a defined specification, for example base stations for a connection specification used mainly for indoor cells like the DECT or WLAN standards. Adaptations of other control nodes in the communication system to the specific specification can be avoided.
Preferably, all messages encapsulating parameter sets to further switching nodes in subsequent handover procedures are sent from the first switching node, i.e. the first node serves as an anchor node. Consequently, the parameters for every handover of a connection between different switching nodes are sent from the node which first served the user equipment in the connection. The embodiment is especially suitable if the parameter sets are generated in the first node from a message originating directly, i.e. without mapping, from the user equipment.
The latter embodiment is also suitable, if the second and the further switching node are identical or collocated and control base station subsystems using different protocols. In this way a mapping in the further handover with a corresponding loss of information is avoided. If the first switching node serves as anchor node of the connection, the signaling load in the communication system is only slightly increased as every subsequent handover is communicated to the anchoring node.
The parameter set encapsulated in the message between the switching nodes corresponds preferably to the protocol used between the second switching node and the second base station subsystem. In this way, the target switching node can relay the encapsulated parameter set without change of the protocol in a minimum time. If for example the handover is performed to a base station subsystem controlled by a BSC, a suitable message is a MAP message encapsulating a BSSMAP massage with the parameter set. In case of a handover to a base station subsystem controlled by an RNC, preferably a MAP message encapsulates a RANAP message comprising the parameter set. Advantages of this embodiment are the short length of the messages between the switching nodes and that all parameter sets can be generated in the first switching node serving the user equipment out of the parameters sent by the user equipment, e.g. in a DTAP message.
Alternatively, the parameter set is sent according to a predefined protocol which can be processed by any switching node in the communication system. If the communication system is updated and a new type of protocol introduced the predefined protocol is a protocol which is already used in all switching nodes of the system. For example, in a GSM system which is updated with UMTS nodes and comprises both types of nodes, the preferable predefined protocol is the BSSMAP protocol. The receiving switching node maps the parameter set to the protocol for the control of the second base station subsystem. Generally, a mapping is performed only in the target node. The embodiment is advantageous if base station subsystems according to an additional communication specification or only few base station subsystems according to a specific communication specification are connected to the communication system because only the switching nodes connectable to these base station subsystems need to perform a mapping. The protocol between the switching nodes needs not to be changed. The flexibility of this embodiment is limited by the parameters in the set according to the default protocol.
In a further alternative, the parameter set is sent for the predefined protocol and a parameter set for a further protocol is encapsulated in an extension field of the predefined protocol if the further protocol is used between the target switching node and the target base station subsystem. It is possible to encapsulate the parameter set for the further protocol in a message according to the further protocol which is in turn encapsulated in the extension field. This simplifies the handling of the parameters in the target switching node. If the predefined protocol is used between the target switching node and the target base station subsystem no parameters for a further protocol need to be encapsulated. As an example, the BSSMAP message comprises an extension field which can contain a RANAP message or the parameters according to a RANAP message. If the predefined protocol is the BSSMAP protocol and the handover is performed to a base station subsystem controlled by an RNC, the MAP message between the switching nodes encapsulates a BSSMAP message with a further RANAP message or a RANAP parameter set encapsulated in the extension field of a BSSMAP message. If the handover is performed to a base station subsystem controlled by a BSC, the MAP message between the switching nodes encapsulates an ordinary BSSMAP message. In this way all necessary parameters are transferred to the second or further switching node at the expense of an increased length of the messages. The target node performs a message extraction or a mapping to the message according to the required specification for the base station subsystem.
Generally, in sets of parameters corresponding to different protocols a first group of parameters have to be set to default values in a mapping between the parameter sets while other parameters are identical in both sets or can be calculated from parameters in the other set without loss of information. To reduce the number of parameters in the extension field and the length of the message containing the parameter set, the first group of parameters according to the further protocol is encapsulated in the extension field. For parameters which are identical in both parameter sets the target switching node performs a mapping from the predefined protocol to the protocol for the control of the second base station subsystem. For parameters which can be calculated from parameters in the other set, the preferable handling depends on the computing expense and additional length of the messages, i.e. these parameters can be either mapped or included in the extension field.
A preferable switching node can be used in a mobile communications system with further switching nodes and user equipment connectable to base station subsystems according to different connection specifications. The switching node comprises means for the control of a first base station subsystem and a first interface for the connection to the first base station subsystem with a protocol corresponding to the connection specification used on the interface between the base station subsystem and the user equipment. A second interface is provided for the exchange of messages with at least one further switching node in a handover procedure of user equipment from the first base station subsystem to a second base station subsystem which is controlled by the further switching node. The message can encapsulate a parameter set for the connection protocol between the second base station subsystem and the further switching node. The switching node has furthermore means to generate the parameter set from a message which is transmitted from the user equipment and means to detect out of this or a different message transmitted from the user equipment whether the user equipment is capable of connections according to different connection specifications on the interface to a base station subsystem. The switching node selects the parameter set encapsulated in the message to the second switching node according to the protocol for the connection of the second switching node to the base station subsystem to which the handover procedure of the user equipment is performed. It is possible to select which set is generated for encapsulation or select a set from several ones generated before. Generally, the switching node comprises a processor system. which allows to embody all of the means described as software programs.
Preferably, the switching node comprises means to generate parameter sets for different protocols and a memory to store the sets. This embodiment allows a reduced handover duration.
Furthermore, a preferable node comprises means to generate parameter sets from a message transmitted from the user equipment or means to map the parameter set for one protocol to a parameter set for a different protocol.
It is proposed that the switching node comprises means to encapsulate a parameter set for a first connection specification into a message for a second connection specification and/or to extract a parameter set for a first connection specification out of a message for a second connection specification. The encapsulated parameter set may be a subset of the parameters necessary for the handover if remaining parameters can be mapped from the message according to the second specification. Correspondingly, messages of the second specification can be used, especially as default message type, without loosing information in mapping procedures and with reduced restrictions from limitations of the second connection specification.
A program unit according to the invention can be stored on a data carrier or be loadable into a switching node for the control of base station subsystems in a communication system. The program unit is preferably part of a program which is executed in the handover of user equipment from a first base station subsystem to a second base station subsystem which is controlled by a second switching node in a communication system comprising base station subsystems differing in the connection specification on the interface to the user equipment. The switching node controls a base station subsystem with a protocol corresponding to the connection specification for the interface between the base station subsystem and the user equipment. The program unit comprises means, e.g. routines or subroutines, to exchange messages in the handover procedure with the second switching node, said message encapsulating a parameter set for one of said protocols and means to generate the parameter set from a message which is transmitted from the user equipment to the switching node. The program unit checks a message transmitted from the user equipment whether the user equipment is capable of connections according to different specifications. It selects the parameter set encapsulated in the message to the second switching node according to the protocol for the connection of the second switching node to the base station subsystem to which the handover procedure of the user equipment is performed. The program unit can perform any steps of the methods described above.